Barack Obama is launching a campaign to persuade American voters that the ailing US economy is safe in his hands. He unveiled a $50bn (£32bn) infrastructure package last night as the countdown began to the mid-term elections in November, in which the Democrats are expected to receive a drubbing.

The president chose the manufacturing town of Milwaukee, home of Harley-Davidson, to announce the scheme which is designed to boost jobs by investing in roads, railways and airport runways. White House officials said the package would run over six years but would be "front-loaded" so that it would jump-start the economy by putting building workers and other manual labourers back to work.

"We're going to rebuild 150,000 miles of our roads – enough to circle the world six times. We're going to lay and maintain 4,000 miles of our railways – enough to stretch coast to coast," Obama said.

The speech, made on the Labor Day holiday that honours American workers, is an indication that Obama intends to focus his efforts almost exclusively on the economy over the eight weeks that remain until the 2 November elections.

His critics – including several representatives of his own Democratic party struggling to hang on to their seats – say this is not before time, accusing the president of having dispersed his energies too widely on healthcare and foreign policy rather than concentrating on voters' fears about their livelihoods.

Obama said that Republicans were hoping Americans would forget the economic policies they put in place that led to the recession and that they had opposed nearly everything he has done to help the economy, and had proposed solutions that had only made the problem worse.

"That philosophy didn't work out so well for middle-class families all across America," Obama told a cheering crowd. "It didn't work out so well for our country. All it did was rack up record deficits and result in the worst economic crisis since the Great Depression."

He said Republicans had consistently opposed his economic proposals and seemed to be running on a slogan of "No, we can't," playing off his 2008 presidential campaign mantra of "Yes we can."

"If I said fish live in the sea, they'd say no," Obama said.

Today marks the unofficial start of the campaign season, and there are signs of growing urgency, if not panic, in Democratic ranks. The Cook Political Report, which monitors congressional races, predicts the Republicans stand to gain at least 35 seats in the House of Representatives – within spitting distance of the 39 needed to regain control – and they are also threatening to recapture the Senate.

A recent Gallup poll gave Republicans a 10-point lead, the largest for the party ahead of the midterms since 1942 and double the advantage it held at the same time in 1994, when it snatched back Congress from Bill Clinton's Democrats.

The infrastructure plan promises to rebuild 150,000 miles of roads, restore 4,000 miles of railway and improve 150 miles of airport runway. It would also pay for the installation of a new air traffic control system and set up a permanent infrastructure bank to channel private and public money into projects.

Obama will announce further job-creating schemes tomorrow in Cleveland, Ohio, including a plan to extend tax incentives for research.

In his weekly speech at the weekend, Obama said that "to heal our economy, we need more than a healthy stock market; we need bustling main streets and a growing, thriving middle class. That's why I will keep working day by day to restore opportunity, economic security and that basic American dream for our families and future generations".

Hilda Solis, the labour secretary, told CBS yesterday that the infrastructure plans would "put construction workers, welders, electricians, back to work – folks who have been unemployed for a long time".

The problem for the Democrats is that none of these initiatives is likely to make a discernible difference before 2 November to the current unemployment rate, which last month crept up to 9.6%.

The White House may even find it impossible to get the new infrastructure scheme passed through Congress before members head back to their constituencies for the vote. Officials admitted yesterday that the first jobs that would be created under the package would not be seen until next year at the earliest.

Meanwhile, the Obama administration's $862bn stimulus package, introduced in the wake of the global economic meltdown, has largely worked its way through the system.

There is also an element of damned if he does, damned if he doesn't. While Democratic candidates complain that Obama has not focused enough on the economy, Republicans suggest he has done too much, portraying him as an insatiable spender of public money.

As an indication of the likely campaign ahead, the Drudge Report, the influential conservative website, led on Monday with the infrastructure package under a picture of Obama and the headline: "Addicted to stimulus – $50,000,000,000 more".

With Obama's presidential approval rating languishing at minus 23%, according to the Rasmussen reports, many Democratic incumbents are openly avoiding any link with him. Some are barely mentioning their Democratic credentials on campaign literature.

The Obama administration says it will avoid piling any further burden on to the national debt as a result of its new economic measures, by balancing the costs with increased tax revenue to be achieved by closing tax loopholes for oil and gas companies and multinationals.